Recent progress in synthesis and electrochemical analysis of room temperature ionic liquids (ILs) has established the promise of this unique class of materials as electrolytes for next-generation Li-ion batteries. ILs are organic salts having melting points below 100° C. and generally consist of a bulky cation and an inorganic anion. The large cation size allows for delocalization and screening of charges, resulting in a reduction in the lattice energy and thereby the melting point or glass transition temperature. ILs have unique physicochemical properties such as negligible vapor pressure, non-flammability, good room-temperature ionic conductivity, wide electrochemical window, and favorable chemical and thermal stability. These properties are desirable for providing IL-based electrolytes for lithium batteries. The vast range of anion and cation chemistries that can be combined to create tailor-made or explicitly designed ILs to complement a specific combination of electrode chemistries also provides a largely untapped materials library that can address concerns regarding battery safety.
Recently, ionic liquids mixed with organic solvents such as ethylene carbonate (EC), diethyl carbonate (DEC) and lithium salts were investigated as thermally stable Li-ion electrolytes (Montanino et al., J Power Sources, 194, 601, 2009). The blending of ionic liquids with conventional electrolytes yielded thermally stable non-flammable electrolytes. However, this work did not address the critical issue of graphite anode protection in the presence of ionic liquids.
The intercalation of Li ions into the graphite basal planes occurs around 0.1 V vs Li/Li+, which is beyond the thermodynamic stability of the organic electrolytes. During this process, the graphite electrode is cathodically polarized to low potential, and electrolyte solvent, salt anions and impurities in the electrolyte are reduced to form insoluble products that are deposited on the surface of the anode to form a passivating layer. This process takes place mostly during the first several cycles of a working battery. Thus, the formed passivating layer, as known as a solid electrolyte interface (SEI) layer (Peled et al., Journal of The Electrochemical Society, 126, 2047, 1979), is crucial for the performance of Li-ion batteries. The nature and behavior of the SEI layer affects the cycle life, rate capability, shelf life and safety of Li-ion batteries. Although ILs are stable at high voltages, their cathodic stability is poor. Thus, one of the challenges is to widen the cathodic stability window of ILs to enable the use of a graphite anode.
The use of pure imidazolium-based ILs as an electrolyte solvent is limited by poor cathodic stability, 1 V vs Li/Li+(Choi et al., Angewandte Chemie International Edition, 51, 9994, 2012). The more cathodically stable ammonium cation-based ILs suffer from co-intercalation of the IL cations into the graphite structure at higher potentials than the Li ion intercalation potential (M. Ishikawa, ECS Transcations, 50(26), 317, 2013; Y. An et al., RSC Advances, 2, 4097, 2012). Recent studies show that pyrrolidinium and piperidinium cation-based ILs exhibit lower reductive potentials than their more popular imidazolium counterparts. These cations also exhibit similar co-intercalation behavior. Maolin et al. (Journal of Chemical Physics, 128, 134504, 2008), using molecular dynamic (MD) simulations of IL on a graphite surface, reported that the butyl group on the imidazolium cation aligned parallel to the graphite surface.
Salem and Abu-Lebdeh (Journal of The Electrochemical Society 161, A1593, 2014) reported the comparison of ionic liquids with different ring sizes of cyclic ammonium cations (pyrrolidinium, piperidinium and azepanium). The disclosure of Salem and Abu-Lebdeh relates to ring size and electrochemical stability. However, they did not find any correlation between ring size and corresponding electrolyte performance in Li-ion cells. Similarly, Belhocine et al., (Green Chemistry 13, 3137, 2011) disclosed alkyl-substituted and ether functional group-substituted azepanium cation-based ionic liquids, but did not contemplate using the synthesized ionic liquids as electrolytes in combination with co-solvents.
These results indicate the importance of understanding the interfacial characteristics of ionic liquids on solid electrode surfaces. Therefore, there is a need to incorporate a novel ionic liquid to form more stable and well-regulated layers at graphite or other anode surfaces of electrodes.